1. Field of the Invention
The present invention relates to a surface acoustic wave device to be used for, for example, band-pass filters and resonators, and a method for manufacturing the same. More specifically, the present invention relates to a surface acoustic wave device having an interdigital electrode made of tantalum, and a method for manufacturing the same.
2. Description of the Related Art
Surface acoustic wave devices have been widely used for band-pass filters and resonators. The surface acoustic wave device used in these apparatuses are strongly required to have good frequency characteristics and very low cost.
An interdigital electrode including gold is provided on a rotation Y-cut quartz substrate so that the angle between the X-axis and the propagation direction of the surface acoustic wave is about 90xc2x0 in the surface acoustic wave device disclosed in Japanese Examined Patent Application Publication No. 61-45892.
Japanese Unexamined Patent Application Publication No. 8-125485 discloses a surface acoustic wave device having an interdigital electrode including tantalum.
Japanese Unexamined Patent Application Publication No. 10-247835 also discloses a surface acoustic wave device having an aluminum layer defining a substrate and a bilayer structure interdigital electrode including a tantalum layer laminated on the aluminum layer.
It is desirable that distribution of a center frequency of the surface acoustic wave device is as small as possible, when a narrow band-pass filter is constructed using the surface acoustic wave device. Accordingly, fine-tuning of the frequencies has been required after completing the manufacturing process of the surface acoustic wave device to be used for the narrow band-pass filter.
A typical method for tuning the frequencies as described above includes fine-tuning the frequencies by dry-etching the interdigital electrode with plasma and a gas including CF4 and O2 (Applied Physics Letters, 30(1), p40 (July 1981)).
For example, while the maximum shift of the center frequency is usually about 300 ppm when manufacturing the surface acoustic wave device having the quartz substrate, it is possible to change the frequency by a maximum of about 500 ppm using the frequency tuning method described above, and the shift of the center frequency is considered to be finally suppressed within about 50 ppm.
The operating frequency of the surface acoustic wave device is generally determined by an equation of f=v/xcex (where v denotes a propagation velocity of the surface acoustic wave and xcex denotes the wavelength of the surface wave). The wavelength xcex depends on the structure of the interdigital electrode, which is usually manufactured by a technique using photolithography. The technique usually includes a combination of a thin-film deposition method such as a chemical deposition method like CVD or a physical deposition method like sputtering or vapor deposition method, and an etching process. These techniques are repeated, if necessary.
When gold is used for the material of the interdigital electrode as in the surface acoustic wave device described in Japanese Examined Patent Application Publication No. 61-45892, a problem occurs in that the adhesive property between the electrode including gold and a piezoelectric plate is insufficient. In addition, gold is so expensive that the material cost becomes too high causing a greatly increased cost of the surface acoustic wave device.
Cost reduction is possible by using tantalum as the material of the interdigital electrode as disclosed in Japanese Unexamined Patent Application Publication No. 8-125485. Since tantalum is a metal having a high melting point, it can be deposited by a physical deposition method such as sputtering. However, since the tantalum film formed by the method as described above has a crystal structure of xcex2-tantalum, its resistivity is as high as about 180 xcexcxcexa9xc2x7cm at room temperature. Accordingly, characteristics of the surface acoustic wave device, especially insertion loss, are worsened because electric resistance of the electrode increases.
Accordingly, a method disclosed in Japanese Unexamined Patent Application Publication No. 10-247835 has been proposed, wherein an aluminum layer having a good electrical conductance is provided on the substrate of the electrode layer including tantalum. However, it is impossible to sufficiently decrease the resistivity of the overall interdigital electrode due to high resistivity of the tantalum film.
In the case of the interdigital electrode having a multilayer structure, an electromechanical coupling coefficient is decreased depending on the metallic material constituting a substrate. As a result, the characteristics of the surface acoustic wave device, especially the insertion loss, are worsened.
Thus, it has been difficult to adjust the frequency shift exceeding 500 ppm by fine-tuning the frequency of the surface acoustic wave device via a dry etching method using plasma. This is because the plasma damages the piezoelectric body, which consequently deteriorates the insertion loss of the surface acoustic wave device.
Stepper exposure equipment is usually used in forming the interdigital electrode. It is impossible in this case to absorb the piezoelectric plate by using the stepper due to stress of the electrode film when the electrode film is deposited, often making it impossible to securely irradiate the light. Consequently, eliminating or preventing the stress described above has been attempted by reducing the deposition rate of the electrode film or by increasing the deposition pressure. However, these attempted solutions to the problems as described above resulted in poor throughput, inferior crystallinity or increased resistivity of the electrode film.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device having an interdigital electrode including tantalum, wherein an adhesive property of the interdigital electrode for adhering to the piezoelectric plate is excellent, the cost as well as resistivity of the interdigital electrode are greatly reduced, and the insertion loss is greatly improved, and also provide a method for manufacturing such a novel surface acoustic wave device.
A surface acoustic wave device according to preferred embodiments of the present invention preferably includes a piezoelectric plate and at least one interdigital electrode provided on the piezoelectric plate, wherein the interdigital electrode includes a first metallic thin film defining a substrate and a second metallic thin film laminated on the first metallic thin film and containing tantalum as a principal component, and wherein at least a portion of the second metallic thin film is xcex1-tantalum.
In one of the novel features of preferred embodiments of the present invention, at least a portion of the first metallic thin film includes titanium.
In another feature of preferred embodiments of the present invention, the first metallic thin film preferably includes a laminated metallic film formed by laminating a plurality of metallic thin films, and at least a portion of the thin metallic film at the side of the laminated metallic film where the second metallic thin films are laminated includes titanium.
The metallic thin film other than the metallic thin film to be laminated with the second metallic thin film of the laminated metallic thin film preferably includes a metal containing Au, xcex1-tantalum, xcex2-tantalum, W, Ag, Mo, Cu, Ni, Fe, Cr or Zr as a principal component.
Preferably, the thickness of the first metallic thin film is about 3 nm or more in at least one preferred embodiment of the present invention.
The first metallic thin film and the second metallic thin film are deposited by sputtering or vapor deposition in the method for manufacturing the surface acoustic wave device according to a preferred embodiment of the present invention.
In the method for manufacturing the surface acoustic wave device according to various preferred embodiments of the present invention, the interdigital electrode preferably includes a multilayer structure having a thin film of tantalum and a substrate thin film to accelerate the thin film of tantalum to be converted into xcex1-tantalum.
While it was known in the art to use tantalum as an electrode of the surface acoustic wave device, use of xcex1-tantalum was not known previously and only the use of xcex2-tantalum was known.
It is known in the art that xcex1-tantalum has low resistance (Sinku Vol. 24, No. 4, p240, 1981).
It is also known in the art that xcex1-tantalum may be deposited when the tantalum film is deposited on a material having a body-centered cubic lattice structure with a lattice constant that is identical or similar to the lattice constant of xcex1-tantalum (Japanese Unexamined Patent Application Publication No. 3-248568).
However, neither use of xcex1-tantalum for the interdigital electrode, nor acceleration of conversion of a tantalum film into xcex1-tantalum when the tantalum film is deposited on the thin titanium film has not been known in the art yet.
The surface acoustic wave device according to preferred embodiments of the present invention includes a first metallic thin film defining a substrate thin film and tantalum, and the interdigital electrode includes a second metallic thin film that is preferentially converted into xcex1-tantalum as will be described hereinafter. Consequently, not only resistivity can be reduced, but also adhesive strength of the interdigital electrode to the piezoelectric plate can be effectively improved, in addition to effectively reducing bending of the substrate after deposition of the electrode. Resistivity of the entire interdigital electrode can be decreased besides reducing the insertion loss of the surface acoustic wave device according to preferred embodiments of the present invention, since the first metallic thin film is formed on the piezoelectric plate defining a substrate, and the tantalum film that is converted into xcex1-tantalum is deposited on the first metallic thin film.
A material such as titanium having an excellent adhesive strength for adhering to the piezoelectric plate can be used for the first metallic thin film, thereby enabling the adhesive strength of the piezoelectric plate to the interdigital electrode to be greatly improved.
Deterioration of characteristics of the surface acoustic wave device under a high temperature and high humidity environment is prevented since xcex1-tantalum is a stable substance.
In addition, bending of the piezoelectric plate after depositing the electrode is prevented by forming the first metallic thin film followed by depositing the tantalum film on the first metallic thin film. Accordingly, the deposition speed of the tantalum film is increased to enable throughput of manufacturing to be improved.
Lowering the insertion loss gives a margin to the insertion loss to enable the tuning range of the center frequency of the surface acoustic wave device to be expanded, or the yield of the surface acoustic wave device to be increased.
Accordingly, preferred embodiments of the present invention provide a cheap and reliable surface acoustic wave device having a wide tuning range of the frequency, wherein the insertion loss is improved and characteristics under a high temperature and high humidity environment are not deteriorated.
The adhesive strength of the interdigital electrode to the piezoelectric plate is improved when at least a portion of the first metallic thin film includes titanium, because titanium has an excellent adhesive property for adhering to the piezoelectric plate. In addition, conversion of the second metallic thin film to xcex1-tantalum is securely accelerated to further reduce the insertion loss, when the tantalum film is deposited on titanium to define the second metallic thin film.
Conversion of the second metallic thin film to xcex1-tantalum is securely accelerated as well to further reduce the insertion loss, when the first metallic thin film includes a laminated metallic thin film, and when at least a portion of the metallic thin film at the side where the second metallic thin film is laminated includes titanium.
When the first metallic thin film includes a laminated metallic film, and the remaining metallic thin films excluding the metallic thin film at the side where the second metallic thin film is laminated preferably include Au, xcex1-Ta, xcex2-Ta, W, Ag, Mo, Cu, Ni, Fe or Zr as a principal component, the electromechanical coupling coefficient is greatly increased to effectively reduce the insertion loss of the surface acoustic wave device.
Conversion of the tantalum film into xcex1-tantalum can be accelerated in addition to effectively reducing the resistivity of the entire interdigital electrode by depositing the tantalum film as the second metallic thin film on the first metallic thin film while depositing the first metallic thin film at a thickness of about 3 nm or more.
In the method for manufacturing the surface acoustic wave device according to preferred embodiments of the present invention, the first metallic thin film and the second metallic thin film are deposited on the piezoelectric plate by sputtering or vapor deposition. Conversion of the second metallic thin film into xcex1-tantalum is accelerated by depositing the second metallic thin film including tantalum as a principal component on the first metallic thin film, thereby enabling the surface acoustic wave device according to preferred embodiments of the present invention to be obtained using a common technique such as sputtering or vapor deposition.